Cessna 120-140 Association

OK, here's everyone's chance to get in your 2 cents worth!

This is a full throttle take-off and climb out to 1,000 ft on a standard day at sea level.

According to the Overhaul Manual, the O-200 maximum RPM is 2750-2800 (Test Operation Limits).
The fuel consumption at full throttle is 51.55 lbs per hour or 8.59 GPH (Test Operating LImits).
(these two are NOT tied together in the Test Operating Limits chart: i.e., fuel consumption at 2750-2800 is 51.55 lbs per hour)

If the propeller restricts RPM to 2600 RPM at full throttle, is the fuel consumption still 8.59 GPH, or is it reduced relative to the lower RPM?

In other words, does the throttle position or the propeller control fuel consumption?

Since an engine is basically a pump, does it have to turn faster to pump more fuel regardless of the fuel available to pump?

Steve A H

8424 wrote: ↑Fri Aug 27, 2021 6:53 amIf the propeller restricts RPM to 2600 RPM at full throttle, is the fuel consumption still 8.59 GPH, or is it reduced relative to the lower RPM?

There should be two fuel curves. One is "propeller load" and the other is "cruise". You will burn more fuel at 2600 in a climb at full throttle than you will at 2600 in level flight at partial throttle.

In other words, does the throttle position or the propeller control fuel consumption?

Yes

Since an engine is basically a pump, does it have to turn faster to pump more fuel regardless of the fuel available to pump?

Two factors determine fuel consumption: manifold pressure and RPM. For a given RPM, the throttle controls manifold pressure. The carburetor delivers fuel based on how much air flows through it, which is a function of the manifold pressure.

So, you can have the same fuel flow at lower RPM and high manifold pressure (climb), and again at high RPM and low manifold pressure (dive).

Now John, that's a great lesson on RPM/Manifold pressure/throttle position/fuel flow!

We all know fuel consumption is higher at full throttle climb out than in cruise for a given RPM. I just was wondering if it maxed out the fuel at 8.59 GPH. The climb/fuel consumption charts for a 1976 150 show a climb from 1,000 to 6000 uses fuel at the rate of 9.33 GPH (9 minute climb using 1.4 Gallons) with no RPM given. It's interesting that Cessna managed to exceed the Overhaul Test Limits full throttle number of 8.59 GPH. Maybe it's the difference between real world testing and crunching numbers.

While folks with constant speed props deal with this each flight, the rest of us just push in the throttle and go with what ever it works out to be.

It really needs to get cooler outside so I can go do something instead of look up stuff!

Steve A H

Interesting question and a great response Thank You . That's why this site is of such value, REAL help answers. I will add 11.5 hrs of HOT STICKY flying ( At Gross ) from northern IL. to convention in GA. and back we used the most fuel ever. 1948 C90 with marvel 95% power 6.64 gal per Hr. average. Did I mention HOT?!!!